The performance of lasers used as optical transmitters in optical communication systems is impaired by reflections at, e.g., the end faces of optical fibers. Light is coupled back into the laser with a different phase position or a different polarization and produces parasitic modes in the laser which result in an undesired change of the emitted frequency or in a reduction of the emitted amplitude.
An arrangement for suppressing such a feedback is disclosed in published German Patent application DE-OS 29 45 466. It consists of a laser and an optical isolator constructed from a polarizer and a birefringent medium. The emitted laser light passes through the polarizer into the birefringenent medium, in which the linearly polarized light is converted into circularly polarized light. This circularly polarized light is coupled into an optical waveguide. Polarization conversion is achieved by arranging that the optical axis of the birefringent medium and the propagation direction of the incident light form a given angle dependent on the material of the birefringent medium, in that case 45.degree.. The thickness d of the birefringent medium in the propagation direction of the light is given by EQU d=Lambda.sub.vak 4.sup.-1 (n.sub.o -n.sub.ao).sup.-1 (1+4K) (Eq. 1),
where lambda.sub.vak is the vacuum wavelength of the incident light, n.sub.o is the refractive index in the direction of the ordinary beam, n.sub.ao is the refractive index in the direction of the extraordinary beam, and K is an integer, in this case zero. A birefringent medium employed in this manner is also referred to as a "lambda/4 layer".
If the laser light having traversed the lambda/4-layer is reflected back into the lambda/4 layer, the circularly polarized light is linearly polarized again, with the plane of polarization of the reflected light being rotated by 90.degree. with respect to that of the incident light. For the reflected light, the polarizer for the incident light thus acts as an analyzer through which the reflected light cannot pass.
From Eq. 1 it is apparent that the conversion of the light from one type of polarization to the other is performed correctly for only one wavelength, and that the lambda/4 layer must have precisely the required thickness. If either of these conditions is not satisfied, the polarization conversion will be incomplete and the reflected light will have a component which will be coupled through the analyzer back into the laser. The prior-art optical isolator is therefore usable for only one particular wavelength.